The rat cellular sequences homologous to two murine sarcoma viruses have been studied in respect to their biochemical behavior and biological function. Studies have been initiated to define the role of type C virus expression on myogenic differentiation in cultured mammalian cells. Several mutant mammalian cell lines have been derived and characterized. Somatic cell hybrid cell lines have been studied in terms of correlating RNA tumor virus expression as a function of phenotypic behavior. Experiments have been conducted to define the interaction between RNA tumor viruses and human herpesviruses. BIBLIOGRAPHIC REFERENCES: Scolnick, E.M., Goldberg, R.J., and Williams, D.: Characterization of rat genetic sequences of Kirsten sarcoma virus: a distinct class of endogenous rat type C virus. J. Virol. 18: 559-566, 1976. Bendas, C.M., Crowell, R.L. and Goldberg, R.J.: The spontaneous release of endogenous rat type C virus from myogenic cells in culture. In Vitro 13: 172, 1977.